I think this will work Lyle. Friend of mine has an -8 with two antennas mounted on a tray under the cowl.
My real reason for answering your question, however, is to point out something I learned when I mounted just one antenna under the cowl: be sure to mount the antennas sufficiently far forward of the firewall. Just how far, I can't say so hopefully someone else will chime in, but at one time I had a GPS antenna under the cowl but only about two inches ahead of the firewall. I found that depending on the direction of flight the weather updates could be very infrequent. For example, one trip from Dallas to Oshkosh a few years ago, there was considerable rain along the route in North Texas and through Missouri. As I approached an area that looked very ominous, my Sirius Satellite Weather had not updated for 45 minutes and I was facing some very dark clouds ahead. My theory is that the satellites are basically in southern orbits and consequently the canopy, roll bar, and firewall, forward baggage area block the antenna from "seeing" the satellites when headed north (low angle). The opposite was true when headed southbound. I ended up in this particular instance circling in order to get an update and eventually landed in Missouri to wait out some of the closer storms. Not long after that trip, I removed the antenna and mounted it back on top of the instrument panel where it was originally. No issues since. I now have two GPS antennas on top of the instrument panel (one ADS-B and one for NAV), right next to each other and no interference between them.
So just a thought to look into how long a shelf to build and how far away from the firewall to place the antennas. Check with some others but IMO I don't think you'll have an issue with interference.
Chris